The goal of this project is the elucidation of the biochemical mechanisms employed by the cell to insure the faithful replication of DNA. Four separate but related approaches will be used: 1 - The influence of DNA primary structure on misincorporation of nucleotides will be assessed by measuring the incorporation of specific nucleotide analogues into DNA on templates of known sequence. 2 - The biochemical mechanisms of six mutator genes which increase spontaneous mutation rates 10-10,000 fold will be investigated by comparing extracts of each to an isogenic parent. Enzymes known to be involved in DNA replication will be compared in these different strains. 3 - The biochemical mechanisms responsible for "error-prone repair" will be studied by comparing DNA replication in extracts of cells treated with chemical and physical agents known to induce mutagenic repair processes. 4 - The pathway of incorporation of the eucaryotic mutagen, N-6-hydroxyaminopurine into DNA will be studied and its mechanims of mutagenesis will be investigated. Studies on the fidelity of DNA replication are fundamental to our understanding of how normal cells grow and divide, and they impinge directly on observations which implicate defective DNA synthesis in several human diseases.